Fix apply_patch path handling and final diff preview

[bstncartwright]

Summary

• resolve patch paths with path.resolve(cwd, filePath) instead of enforcing cwd containment
• allow absolute paths, parent traversal paths, and symlinked paths outside cwd
• retain the computed diff preview on final tool results so completed patches render with changed-line previews
• update tests for outside-cwd path handling and final preview rendering

Closes #21.
Closes #23.

Validation

• npm test
• npm run check
• git diff --check